The present invention relates to a rotating irradiation apparatus, and more particularly to a rotating irradiation apparatus suitable for irradiating an affected area with an ion beam.
An ion beam irradiation system is a particle radiotherapy system used to irradiate an affected area with a proton beam or an ion beam of carbon ions, etc. to treat cancer. The ion beam irradiation system includes a rotating gantry, which is a rotating irradiation device. For example, the rotating gantry includes a front ring, a rear ring, and a gantry body connected between the front and rear rings, as indicated in Japanese Patent Laid-open No. 11-47287. The gantry body is provided with an irradiation device (or irradiation nozzle) and a beam delivery device (or beam path) for guiding an ion beam. The front and rear rings are supported by their respective radial support devices each including a plurality of rotatable rollers. More specifically, each roller provided in each radial support device supports the front or rear ring. Some of the rollers supporting the rear ring are rotated by a motor to rotate the rotating gantry. This rotation of the rotating gantry contributes to directing the irradiation device in an ion beam irradiation direction to irradiate an affected area with an ion beam.
Japanese Patent Laid-open No. 2000-140134 discloses a rotating gantry for a particle radiotherapy system, in which the rear ring is supported by a support device including rollers, and the front ring is supported through a rotating ring by the front support frame mounted on the base portion. The rotating ring allows the front ring to be rotatably mounted on the front support frame. The front support frame, in turn, is mounted on a leg portion rotatably connected to the base portion by pins. These pins enable the leg portion to be tilted in the direction of the rotational axis of the rotating gantry. Therefore, the rear ring also can be tilted in the direction of the rotational axis of the rotating gantry. A bed is mounted on the front support frame. This arrangement can prevent misalignment between the bed and the rotating gantry.
Japanese Patent No. 3599995 also describes a rotating gantry for a particle radiotherapy system. This rotating gantry includes two rotating rings each supported by rollers. Further, in order to maintain the rotational center position of this rotating gantry within a predetermined tolerance over an extended period of time, at least one of these rotating rings is rotated by a pair of drive rollers that sandwich the rotating ring and that are in contact with respective sides of the rotating ring.
The particle radiotherapy system is designed to irradiate a target position on an affected area with an ion beam for treatment. To increase the accuracy of treatment, it is desirable that the irradiation position does not change as the rotating gantry rotates. In reality, however, as the rotating gantry rotates, the ion beam irradiation position moves three-dimensionally in a whirl due to the distortion of the rotating gantry caused by the weight of the deflection electromagnet, etc. provided on the rotating gantry. The center of this three-dimensional whirl of the irradiation position is referred to as the “isocenter” or “irradiation target center”. One of the fundamental performance requirements for the particle radiotherapy system is that the ion beam irradiation position only moves, or whirls, within a sphere with a diameter of a few millimeters centered at the isocenter as the rotating gantry rotates.
One method for maintaining the ion beam irradiation position close to the isocenter to thereby satisfy the above requirement is to reduce the distortion of the gantry body connected between the front and rear rings by constructing it such that it has a highly rigid structure. This greatly reduces the three-dimensional whirling of the ion beam irradiation position.
There is another factor in causing an error in the irradiation position. This factor cannot be removed by increasing the rigidity of the rotating gantry body. For example, if the rotational axes of the rotatable rollers of the radial support devices are fully parallel to those of the front and rear rings, the rotating gantry does not move in the axial direction on its own when it rotates. In reality, however, the rotational axes of the rollers of the radial support devices are not fully parallel to those of the front and rear rings due to the following factors: (a) errors in the machining and assembly of components of the rotating gantry; (b) adjustment errors during installation of the rotating gantry; (c) deterioration of the building with age; and (d) small variations in height, etc. due to wear of the rings and rollers. Thus, there is a certain degree of misalignment between the rotational axes of the rollers and the rings. As a result, the gantry tries to move in the axial direction due to the axial component of the rotational force applied to the engagement surfaces of the rollers, as shown in FIGS. 1A to 1C. (This movement is referred to as a “skewing movement”.) Assume that a roller A (corresponding to a rear ring 20 described later) is rotating in contact with a roller B (corresponding to a roller 22 described later) having a smaller diameter than the roller A, as shown in FIG. 1A. In this case, if P denotes the weight of the roller A and μ denotes the friction coefficient between the rollers A and B, then the roller rotational force produced in the direction as indicated in FIG. 1A is P*μ. If the rotational axis of the roller A is displaced by an amount e as shown in FIG. 1B due to the reasons stated above, then the axial component of the roller rotational force P*μ is P*μ*θ, as shown in FIG. 1C (where θ denotes the angle at which the rotational axis of the roller A is displaced from that of the roller B). This axial component force P*μ*θ causes the roller A to move in the axial direction.
The radial support devices described in Japanese Patent Laid-open No. 11-47287 are fixed to the mounting base portion, which prevents them from moving in the axial direction of the rotating gantry. As a result, the rotating gantry itself moves in the axial direction or a horizontal direction as it rotates, resulting in increased three-dimensional whirling of the irradiation position.
In one method for reducing the movement of the rotating gantry in the axial direction, the rotating gantry is mounted between two thrust support devices. These thrust support devices press the outer sides of the front and rear rings against the rotating gantry in the opposite axial directions to restrain the movement of the rotating gantry in the axial direction. With this method, however, the thrust support devices receive large reactive force, making it necessary to considerably increase their rigidity. Furthermore, the larger the axial thrust force applied by the thrust support devices, the larger the friction between the rings of the rotating gantry and the thrust rollers of the thrust support devices and hence the larger the required rotational driving force. Further, an increase in the axial thrust force results in an increase in the wear of the thrust rollers of the radial support devices and the front and rear rings, promoting the movement of the rotating gantry in the axial direction.
In another method for reducing the movement of the rotating gantry in the axial direction, the rollers of the radial support devices do not have the ability to rotate the rotating gantry. Instead, the apparatus is provided with a mechanism for performing both a rotational driving function and a braking function, as indicated in Japanese Patent No.3599995. Thus, to reduce the “skewing movement” of the rotating gantry, the radial support devices are dedicated to supporting the weight of the rotating gantry, while the drive mechanism rotates the gantry. However, this method still cannot prevent occurrence of skewing due to the factors (a) to (d) above.
For example, in the case of the structure shown in FIG. 4 of Japanese Patent No. 3599995, gear teeth are formed on the peripheral portion of a rotating ring and engaged with the pinion coupled to the motor to rotate the rotating gantry. In this structure, however, there is a backlash in the mesh between the gear teeth and the pinion even right after they are installed, and furthermore this backlash increases with time due to their wear. As a result, it is difficult to maintain sufficient rotation angle accuracy of the rotating gantry (for example, a stopping accuracy of ±0.25 degree or better). Further, there is an increasing need for an inching function to adjust the rotational position of the rotating gantry by rotating the gantry by a small angle based on positional information obtained from an image of the affected area portion after positioning of the rotating gantry at a predetermined angle. However, due to the above backlash, the rotation angle of the rotating gantry needs to be adjusted each time the rotational direction of the gantry is reversed, making it difficult both to achieve a rotation angle accuracy of ±0.25 degree or better without spending time and to achieve the inching function.
Skewing occurs due to the factors (a) to (d) above even with the gantry structure shown in Japanese Patent Laid-open No. 2000-140134, as well as with the structure shown in FIG. 1 of Japanese Patent No. 3599995 in which drive rollers are brought into contact with the sides of the rotating rings to rotate the rotating gantry.
In order to increase the irradiation beam positioning accuracy of the rotating gantry (that is, to maintain the irradiation position close to the isocenter), and maintain the increased accuracy, it is important to provide the rotating gantry with capabilities to remove the factors (a) to (d) above.
Specifically, the capability to accommodate dimensional errors due to the factors (a) and (b) as much as possible allows accurate alignment to be easily achieved. Further, the ability to quickly detect and easily correct misalignment between the rotational axes due to the factors (c) and (d) (or changes that may slowly occur over a long period of time) allows reducing the installation adjustment and maintenance time of the rotating gantry. Therefore, a rotating irradiation apparatus employing a rotating gantry with these capabilities can achieve high throughput.